Basic Breast Cancer Facts One in eight women in the United States will develop breast cancer in her lifetime. Male breast cancer can occur at any age, but is most common in older men. Breast cancer is caused by an uncontrolled growth of breast cells. Often there are no symptoms of breast cancer, but the first sign is typically a lump or abnormality in the breast. Symptoms can include breast pain, nipple discharge other than breast milk, a lump under the arm, swelling of all or part of the breast, or skin irritation. The most significant risk factors for breast cancer are age, gender, and medical or family history. Simple risk factors that can be controlled are diet, weight, exercise, stress and anxiety.